The present invention relates to an apparatus and method for controlling the size distribution of gel microspheres made from aqueous dispersions, i.e., sols or solutions that form sols, so that a majority of the microsphere population formed will be in a preselected and relatively narrow size range.
Current gel microsphere processes produce a product of wide variability in size range. This variability increases as microsphere diameter decreases, particularly for microspheres with diameters below 300 microns. Efforts to control this undesirable variability of size range of gel microspheres include the process described in commonly assigned U.S. Pat. No. 3,617,585 which issued Nov. 2, 1971 in the names of P. A. Haas and S. D. Clinton. Table II of the aforementioned patent shows an improved control in size range variability obtained by the process as compared to another sol-gel process. While this patented process provided a substantial improvement in narrowing the size-range distribution of microspheres, the reported scatter from mean diameter of these microspheres significantly detracted from the overall production efficiency and economics of said process because of unwarranted waste when applied to making microspheres requiring a narrow size-range variability of 300 microns or below.
One specific application having critical need for product microspheres of a narrow size-range variability is microspheres incorporating values of uranium, thorium, plutonium, and combinations thereof intended for vibratory-packed columns of nuclear reactor fuels. According to recent reports on diversion resistant processes for fuel refabrication, effective nuclear performance in such columns is attained by three size ranges of microspheres of approximately 40:10:1 diameter-to-diameter ratios. See, for example, W. J. Lackey, et al, "Assessment of Gel-Sphere-Pac Fuel For Fast Breeder Reactors", ORNL/TM-5468, Oak Ridge National Laboratory (1978) and P. A. Haas, et al, "Chemical Flowsheet Conditions For Preparing Urania Spheres By Internal Gelation", ORNL/TM-6850 (1979).
The foregoing technical reports provide detailed information about processes for gel microsphere formation from aqueous dispersions, subsequent treatment procedures, and ultimate loading into fuel columns for nuclear reactors and are, therefore, incorporated by reference. Additional applications for controlled microspheres exist in other fields such as structural ceramics and spherical catalysts.
The simplicity and adaptability of the present invention make it amenable to current uses of sol-gel processing such as remote operations for preparing nuclear-fuels or wastes, and other uses mentioned above. Preferably, the subject method is carried out on a continuous basis to achieve commercial scale quantities, but batch processing of such quantities is also possible. Downtime for maintenance or replacement of components is substantially obviated by the simple design of the present invention as compared to the heretofore utilized formation devices.
An important aspect of our invention lies in the discovery that the wide size-range variability of prior art processes, such as above-described, for forming sol-gel microspheres can be explained by particle kinematics of droplets exposed to a turbulent-flow field controlled only by hydraulic action and surfactant additions. Classical fluid flow theory suggests three separate regions of flow behavior for well-developed flow. These are laminar flow at the walls, a transition layer of limited eddy activity adjacent thereto, and an inner turbulent core of violent eddy activity. Since hydraulic activity has been identified as controlling, it is apparent that droplets flowing therein will have variant operative forces of formation and hence different diameters.
The functional relationship between droplet and ultimate microsphere size in sol-gel processes has long been appreciated by the art. Heretofore, the principal approach to smaller droplets has been finer restrictions in orifices, capillaries or related openings in formation devices. These equipment modifications have done little to produce microspheres of narrow size-range variability. Increases in waste because of unacceptable product and even lower production rates of microspheres are characteristic of such modifications and are, therefore, undesirable.
Interfacial tension between the dispersed fluid and solvent of prior art processes is controlled by hydraulic action and addition of surfactant concentrations to the dispersion. Hydraulic action appears to be the more important of these, thus leading to the conclusion that droplet size is flow rate controlled with greater flow (N.sub.re &gt;200) producing finer droplets. One researcher, A. J. Karabelas, in "Droplet Size Spectra Generated In Turbulent Pipe Flow Of Dilute Liquid/Liquid Dispersions", AIChE Journ., 24, pp. 170-180 (1978), AIChE, New York, N.Y., has even predicted droplet size based on Weber Number for water/petroleum-distillate dispersions.
Thus, there is a need for an improved and more efficient method and apparatus for producing close size-controlled microspheres of below 300 microns diameter while maintaining a high degree of the requisite uniformity essential for retentive strength and resistivity. The improved method and apparatus will produce microspheres of suitable size-range fields for direct deployment to the ultimate application thereby eliminating corrective procedures for removing defective portions or separating under and over-sized fractions of the final product. Of course, a significant increase in production volume and a marked decrease in waste will also follow.